This invention is in the field of vascular grafts made of synthetic fibers that may be used in a variety of vascular surgical procedures. Particular interest herein is to grafts used to replace occluded portions of arteriosclerotic vessels or used to form new blood pathways in a vascular reconstruction procedures as generally disclosed in U.S. Pat. Nos. 4,517,687, 4,047,252, 4,530,133, and 3,878,565 and in applicants' co-pending application Ser. No. 068,662 which issued on Mar. 28, 1989 as U.S. Pat. No. 4,816,028.
There are basically three kinds of prior art grafts, namely extruded, knitted and woven. Extruded grafts have the desirable characteristics of being generally strong and nonporous which precludes subsequent hemorrhaging. Unfortunately, this absence of porosity prevents tissue ingrowth following implantation, such ingrowth being considered desirable. Also these grafts are relatively stiff and nonconforming and thus are difficult to handle, suture and implant.
Knitted grafts have certain advantages over extruded grafts, namely porosity, flexibility, softness and a structure adaptable to have a velour type outer surface. Accordingly, they conform easily to blood vessels, which reduces surgeon's and patient's time in the operating room; the porosity and velour surfaces allow considerable endotheliazation (tissue ingrowth). The principal disadvantage of knitted grafts is that the porosity is so great, hemorrhaging will occur unless the graft is preclotted.
Preclotting is a separate step whereby the graft is immersed for about fifteen minutes in a quantity of about 100-150 ccs. of the patient's own blood, after which the graft is allowed to stand for clotting to occur. Preclotting substantially prevents hemorrhaging, while allowing subsequent tissue ingrowth; however sometimes preclotting is not permissible, as where the patient has been anti-coagulated or has bleeding diathesis. In these cases a knit graft requiring preclotting cannot be used. A further negative consideration about knitted grafts is that they dilate and stretch more than other types.
Another known knitted graft is coated in manufacture with albumin to prevent initial bleeding following implantation. Other prospective coated grafts will use collagen or gelatin for the same purpose. The albumin coating obviates the necessity for typical preclotting during surgery; however, such coating renders the tubular graft stiffer and more difficult to handle during implantation. Also such a coated product is considerably more expensive than a simple woven graft. A further disadvantage of an albumin-coated graft is that it must be prepackaged in saline.
Woven grafts have certain advantages over both extruded and knitted grafts. In woven grafts porosity is lower so that preclotting is not required. Woven grafts have the disadvantage of being relatively stiffer and less conforming than knit grafts, and thus they are more difficult and time consuming for surgeons to use. A further disadvantage with known woven grafts is a tendency for the end edge of a tubular graft to fray when cut. Tubular grafts may be cut perpendicular to the axis of the tube producing a generally round end opening or cut oblique to the axis producing a generally oval end opening. With a perpendicular cut there is a tendency for fraying of weft or pick threads since they are exposed at the end of the graft. Such fraying has been substantially reduced by the use of periodic double leno warp threads within the overall weave as described in applicants' copending application, Ser. No. 068,662 which issued as U.S. Pat. No. 4,816,028. Oblique cuts of the graft are often preferred to perpendicular cuts because the resulting greater cut surface area to be sutured adds security; also, the oblique junction avoids an abrupt direction change and thus permits smoother blood flow. Unfortunately, oblique cut graft ends have a tendency for fraying of both warp and pick threads for which the double leno weave is not a fully satisfactory solution. Any fraying of the cut edge not only causes delays during surgery, it may render the graft unsafe and unusable.
The above description of prior art grafts shows some of the numerous parameters considered in the selection of vascular grafts. Additional factors include tissue compatibility, nonthrombogenicity of the surface, deterioration of the graft with time, resistance to infection and resistance to kinking at the joints of the patient.
Typically in the manufacture of both knitted and woven tubular grafts, after the tubular body is formed it is crimped to form circumferential corrugations or ribs that provide strength and resilence against kinking and collapsing of the tube or narrowing of the lumen from bending or twisting. Known woven grafts often use a polyester yarn such as Dacron.RTM. (polyethylene terephythalate) yarn, Type 56 made by E. I. Dupont Corp. Such yarn has been designated 40 denier/27 or 70 denier/34, where 40 denier/27 or 40d/27, for example, represents 40 grams of weight per 9000 meters of yarn and comprises 27 filaments, or 1.48 denier per filament. Dacron.RTM. is a registered trademark of E. I. Dupont for polyester yarn. The selection of 40 d/27 yarn in single or double ply as the standard of the industry has been dictated by what was available on the market and what has been approved by the F.D.A. Prior patents referred to above describe more fully this standard yarn, which may also be texturized in a standard way. i.e. false twisting the yarn fibers at a spindle speed of about 250,000 rpm, under 8 to 15 grams of tension at about 450.degree.F. Double ply yarns cited above are formed by twisting together two texturized yarns at a twist of about one half turn per inch.
The weaving of arterial grafts is done on known weaving apparatus with a matrix of warp threads through which are woven weft or fill threads. By incorporating a twill or velour weave, velour loops are produced in certain warp threads on the inner and/or outer surfaces of the woven fabric. As is known, these twill threads are preshrunk, so that upon the subsequent shrinking of the completed woven graft tube made of otherwise unshrunk yarn, all threads will shrink except the velour ones which will extend outward from the surface as loops adapted to receive tissue ingrowth.
In view of the above-described function parameters and the differences between the various prior art grafts, compromises in characteristics are required with each selection. More specifically, if one wishes softness and pliability and porosity, the choice must be a knit graft with the required preclotting.
In applicants' co-pending application, Ser. No. 068,662, which issued as U.S. Pat. No. 4,816,028, an invention was disclosed that provided a graft of a woven fabric with softness and pliability of a knit fabric and the low porosity of prior art woven fabric, thus avoiding the high porosity and preclotting requirement of knit and the stiffness of prior woven grafts. Such new fabric utilized various weave patterns (particularly double lenos in place of certain warp threads), where each leno consists of a pair of warp threads twisted or crossed one over the other between successive fill threads, and a double leno consists of a pair of lenos. Between sets of lenos is a set of plain/twill threads, each set comprising, for example, four warp threads alternating as plain, twill, plain, twill. Typically, a twill designated "3/1", means the fabric is woven so that the twill thread lies "over" three successive fill threads, then "under" the next fill, then over the next three, etc. The alternating plain threads designated "1/1" means simply "over" one fill, "under" the next fill, over the next, and so on.
The lenos are effective in reducing fraying or unraveling of a graft, particularly where the cut is perpendicular to the tubular axis of the graft. As discussed above, these lenos, while helpful, do not provide a fully satisfactory solution for extreme oblique cuts at the tubular end of a woven graft. Even a small amount of fraying is bothersome to a surgeon; significant fraying causes serious concern for the security and patency of the suture line. Such concern has led to the present invention of a new and substantially different and better weave pattern for the fabric of the graft, regardless of what combination of plain, twill and/or leno threads it has, as discussed below.